1. Field of the Invention
The present invention relates to optical communication, and, more specifically, to optical channel monitors in dense wave-division multiplexing systems.
2. Description of the Related Art
Historically, the fibers in optical communications systems were illuminated with light consisting of one, or at most a handful, of wavelengths. With the widespread adoption of dense wave-division multiplexing (DWDM) technology, it is now common to light fibers with tens or even hundreds of different wavelengths simultaneously, each wavelength representing a different channel within the system. As the number of wavelengths and the general complexity of these systems have increased, correspondingly higher demands have been placed on the optical performance monitoring systems that are used to manage bandwidth, power, amplification, attenuation, and dynamic filtering within these systems. These optical performance monitoring systems and the system attributes they control are essential for robust operation of the network. They are also key elements of fault reporting, analysis, and management subsystems.
One of the primary functions of optical monitors in optical communications systems today is the detection of the channels that are present in the various optical links of these systems. The conventional approach to channel detection used by these optical monitors is based on certain assumptions about the optical spectrum of a DWDM signal. A typical DWDM signal spectrum consists of sharp peaks located at the centers of the channel wavelengths superimposed on a smooth background of additive spontaneous emission (ASE) noise. This is illustrated by FIG. 1, which depicts power (dBm) versus wavelength (nm) for a DWDM signal composed of two channels 102 and 104 superimposed on an ASE noise background. Note that the peaks of the spectrum 102 and 104 correspond to the channels in the signal at typical International Telecommunications Union (ITU) standard 100-GHz grid channel-to-channel spacing in the vicinity of 1593 nm. Under these conditions, channels can be detected by finding the peaks in the spectrum. This approach works even in the presence of relatively strong ASE noise.
However, modern DWDM systems include additional components, notably optical add-drop multiplexers (OADMs), wavelength interleavers, and active optical switching and multiplexing components, which generally employ sharp wavelength filtering to accomplish their various functions. As DWDM signals pass through these components, the components' filtering functions are impressed on the smooth ASE noise background of the signals, resulting in sharp spectral features centered on and about the ITU channel locations. This is illustrated by FIG. 2, which depicts power (dBm) versus wavelength (nm) for a DWDM signal that has passed through various filtering components. This signal includes three channels 202, 204, and 206 superimposed on an ASE noise background. Note that the peaks of the spectrum corresponding to channels 202, 204, and 206 are not significantly higher, in some cases, than the peaks of the filtered ASE noise. In particular, in FIG. 2, some of the noise peaks 208 corresponding to the shorter wavelengths have more power than some of the channel peaks. Under these conditions, the peak-finding approach to channel detection is often inadequate. With increasing modulation rate and filter cascading, the width of the peak is not a clear indicator of the presence of a channel. Although it is still possible to distinguish the channel peaks from the interleaved ASE peaks given sufficient spectral resolution in the spectrometer within an optical monitor, such spectrometers are relatively expensive and have slower scan speeds than peak detection devices.